Technical Field
The present invention relates to a silica glass member and a method of manufacturing the same. More specifically, it relates to a silica glass member for photomask which can be suitably used in photolithography in a vacuum ultraviolet wavelength region.
Description of the Related Art
In recent years, in the lithography technology, a demand for fine patterning in semiconductor devices has increased more and more, and a method has been employed in which the numerical aperture of a lens to be used for exposure is increased by shortening of the exposure wavelength or an immersion lithography technique to immerse pure water or the like between the lens and the wafer.
The resolution R in photolithography can be expressed by the expression R=k1λ/NA, where λ denotes the wavelength of exposure light, NA denotes the numerical aperture representing the lens performance of the exposure apparatus, and k1 denotes the process constant, and the resolution can be improved by shortening the exposure wavelength λ, increasing the numerical aperture NA, and decreasing the process constant k1.
Here, with regard to the exposure wavelength λ, g-rays (436 nm) of a mercury lamp was first used and i-rays (365 nm), a KrF excimer laser (248 nm), and an ArF excimer laser (193 nm) have been so far used, and the wavelength of the light source has been shortened.
The numerical aperture NA geometrically represents the size of the lens, and it is expressed by the expression NA=n·sin θ, (where n denotes the refractive index of the medium between the lens and the wafer and θ denotes the angular aperture of ray of light) in a case in which the exposure light is focused by the lens and an image is formed on the wafer surface.
Here, when an ArF excimer laser having a wavelength of exposure light of 193 nm is used and the immersion lithography technique is used, a resolution of 43 nm can be achieved in a case in which the numerical aperture is set to 1.35 and the process constant k1 (k1 factor) is 0.3.
Moreover, a silica glass substrate is suitably used as a substrate for photolithography using this ArF excimer laser since it exhibits low thermal expansion property and excellent optical transparency.
Examples of the performance required of the silica glass substrate may include light resistance that the silica glass substrate does not deteriorate optical transparency even when being exposed to high energy light in the case of using an ArF excimer laser. In addition, in the case of conducting immersion lithography, the angular aperture of ray of light increases since the difference between the refractive index of pure water present between the lens and the wafer and the refractive index of the resist decreases, and a problem is thus caused in the effect of polarization. Hence, the silica glass substrate is required to exhibit low birefringence. This is because there is a case in which the transmitted exposure light undergoes polarization change and the imaging performance deteriorates when the silica glass substrate exhibits birefringence.
For example, JP 2001-180963 A discloses a method in which a porous silica glass body (soot) is fabricated by flame hydrolysis of a silica glass forming raw material, a silica glass ingot is then fabricated by the VAD method by which the porous silica glass body (soot) is transparentized and further subjected to a heat treatment in a hydrogen atmosphere to be doped with an OH group and hydrogen, thereby improving the light resistance to an ArF excimer laser and the like as a method of manufacturing silica glass satisfying these requirements.
In addition, JP 2002-316831 A discloses a method of manufacturing fluorine-doped silica glass in which a porous silica glass body (soot) is fabricated by flame hydrolysis of a glass raw material for forming silica glass and then subjected to dehydration, fluorine doping, and a transparentizing treatment, thereby improving the transmittance and laser resistance of the fluorine-doped silica glass to vacuum ultraviolet light having strong energy such as an F2 excimer laser. In the fluorine-doped silica glass described in JP 2002-316831 A, it is considered that the thermal expansion at near room temperature is decreased by about 10% as compared to the case of only quartz glass by fluorine doping.
In addition, JP 3228676 B discloses a method in which the porous silica glass body (soot) is subjected to zone melting at a degree of vacuum of 100 Pa or less to be formed into transparent glass and then subjected to treatment to determine the fictive temperature in an atmosphere of an oxygen-containing gas or a hydrogen-containing gas, thereby retaining an excellent transmittance at a wavelength of 165 nm even after far ultraviolet irradiation.
Meanwhile, as a means for modifying silica glass, for example, JP 2006-225249 A discloses a method in which relaxation of the glass structure is promoted and birefringence is decreased by subjecting the silica glass to an annealing treatment under specific conditions as an additional treatment after manufacture, and hydrogen doping is conducted by changing a part of the annealing treatment step to a hydrogen atmosphere, thereby improving the light resistance.
It is required to use a double patterning method in order to achieve a resolution of 43 nm or less by an exposure method using an ArF excimer laser. Double patterning is a method in which exposure is conducted by being divided into two times, and it is also possible to achieve a resolution of 32 nm or less, which is a finer device pattern by using this method.
In the case of conducting fine patterning of the device by using double patterning, a significantly high overlay accuracy of patterns is required between the two times of lithography since aberration of patterns occurs when the position of the target pattern is not accurately exposed.
Hence, the silica glass substrate for photomask is required to exhibit lower thermal expansion as compared to conventional silica glass in order to avoid position aberration due to thermal expansion at the time of exposure. Here, the overlay accuracy of double patterning exposure refers to the sum of the overlay accuracy for the two times of exposures, and the overlay accuracy required for each exposure is said to be about from 3 to 4 nm. Meanwhile, the thermal expansion coefficient of ordinary silica glass is from 5.0×10−7/K to 6.0×10−7/K and the elongation of a 1 cm quartz piece is from 5 to 6 nm per 1 K of temperature rise, and it is thus hard to say that it is sufficient as the required accuracy. Hence, the silica glass substrate for photomask is required to exhibit lower thermal expansion than ordinary silica glass.
In addition, as a silica glass substrate for light transmission type photomask, the optical transparency at 193 nm of the exposure wavelength of the ArF excimer laser is required to be equivalent to that of the conventional silica glass.
As silica glass characterized by low thermal expansion property, ULE glass (Corning Code 7972) manufactured by Corning Incorporated and the like are already known (“Amorphous Materials”, Paper presented to the Third International Conference on the Physics of Non-Crystalline Solids held at Sheffield University, September 1970). In the “Amorphous Materials”, it is reported that thermal expansion is decreased by doping silica glass with TiO2, and a significantly low thermal expansion coefficient of 0.1×10−7/K or less is exerted by adjusting the concentration of TiO2. However, in the TiO2—SiO2-based glass, the optical transparency at 193 nm is significantly poor since the absorption edge of the ultraviolet wavelength is present at from 300 nm to 400 nm, and it is thus impossible to achieve both low thermal expansion property and optical transparency (Journal of Non-Crystalline Solids, Vol. 11 (1973) p. 368). This is because absorption in the visible light region occurs by the so-called d-d transition between energy gaps in the d electronic structure of Ti by doping TiO2. In addition, it is known that the absorption in the visible light region by a Ti ion is affected by the adjacent oxygen atom, and it is known that the absorption wavelength varies depending on the ionic valence of Ti3+ and Ti4+, but the absorption band is formed between 300 nm and 400 nm in any case. Hence, development of a silica glass substrate by a method other than the TiO2—SiO2-based glass is required.